The resolution of display panels are getting higher and higher. This results in large amounts of source driver outputs in the driving circuit. Each of these source driver output includes a source amplifier. A FHD (Full High Definition 1920×1080) RGB display driver IC includes 3240 source amplifiers. Variation among these amplifiers would result in display non-uniformity.
One of the major causes of variation is the input offsets of the amplifiers. Conventional design methodology to reduce input offset is to increase the device sizes and to increase the static bias current. In contrary, in high resolution display driver ICs, as more and more source amplifiers have to be squeezed, there is strong demand to reduce the device sizes and to reduce the bias current of each amplifier. For very high resolution displays, the number of source amplifiers is so large that they cannot reside in a single display driver IC. Thus, two or more display driver ICs cascade together to drive a single display panel. This leads to further challenge to maintain uniformity among the source amplifiers in different display driver ICs.
Chopping is a method to overcome input offset problem of amplifier circuits. Referring to FIG. 1, the two configurations of the amplifier circuit would produce input offsets in opposite directions with almost the same magnitude. By repeatedly alternating the two directions, the average value of the amplifier output voltage would be much closer to the input voltage. In particular to LCD displays, for the nature of liquid crystal which may only be driven by a waveform of zero DC component, a bipolar driving waveform is used (as shown in FIG. 2), “+” and “−” denote the two polarities. The polarity changes once for each frame, thus the average (DC) voltage is zero. Other display technologies may not have this requirement, e.g. AMOLED may be driven by a waveform consisting of a DC component. “+VH” and “−VH” denote the 2 voltage levels of the 2 driving polarities for VH grayscale level pixels. “+VL” and “−VL” denote the 2 voltage levels of the 2 driving polarities for VL grayscale level pixels.
Chopping Pattern throughout this specification refers to the way how the source amplifiers X-Y chopping circuit configuration alternates in each row and each frame. It can be denoted by multiple frames and each frame as a matrix of ‘X’ and ‘Y’ characters. Column inversion is a scheme that all pixels of a column of the display have the same driving polarity and the pixels of the neighboring columns have the opposite driving polarity.
FIG. 3 shows a chopping pattern with column inversion of driving polarity. Referring to FIG. 3, “+” or “−” denotes the driving polarity. Each pixel would undergo all four combinations of +X, −X, +Y and −Y in some different sequences. So, in average the brightness levels of the pixels of the same greyscale level appear to be uniform. Pixels of opposite chopping directions X and Y are well mixed (high spatial domain frequency.) It does not cause flickering for most images. However, some images driven by this chopping pattern would introduce low spatial domain frequency components by some pixels of the same greyscale level. These images are killer pattern images to this chopping pattern. Another chopping pattern could be devised to accommodate this image. However, such chopping pattern is also subjected to flickering problem with some other images.
FIG. 4 shows a killer pattern image example to a chopping pattern, which is alternating single-pixel width lines of two different greyscale levels. Referring to FIG. 4, coincidently, the image matches the chopping pattern such that all VH greyscale level pixels in a large area of the image have the same chopping directions in all frames (low spatial domain frequency): chopping direction X in Frame 1 and Frame 2, direction Y in Frame 3 and Frame 4. Similarly pixels of VL greyscale level also have the same problem. Flickering in ¼ of the frame refresh frequency becomes observable.
The input voltage of source amplifiers come from the gamma circuit. There are amplifiers (Gamma amplifiers) in the gamma circuit. The outputs of the gamma circuit in a two or more cascade display drivers can be slightly different due to input offsets of these amplifiers. Therefore chopping technique may be applied to these amplifiers so that the average output voltages of the gamma amplifiers in these display drivers are much closer. Thus, the chopping pattern covers both gamma amplifiers and source amplifiers. However, such chopping are subject to flickering problem with killer pattern images, the same as source amplifier chopping as aforementioned.